1. Technical Field
The present invention relates to curtain walls used for building exteriors and, more particularly, but not by way of limitation, to methods of and systems for constructing and assembling curtain walls incorporating mullion mounting tongues having integrally formed screw-race configurations.
2. History of Related Art
The use of curtain walls for building construction is wide-spread and generally accepted by municipal building standards. They are cost effective and often aesthetically appealing. Curtain walls are typically constructed of extruded aluminum frame support members for supporting a plurality of panel members that serve as the exterior of a building. Such panel members are most often panes of glass, and often double pane glass sections, but other paneled building materials such as aluminum, granite, slate, or concrete are also utilized. Such panel members are often of identical size and shape.
In the assembly of a curtain wall, the vertical and horizontal mullions are first secured to the building superstructure in a “grid” type array. The panels that must be assembled to the grid formed by the vertical and horizontal mullions are likewise assembled thereto and carefully sealed to prevent water infiltration. These panels are often made of glass and are secured in place by pressure plates bearing thereagainst. The pressure plates are assembled to the curtain wall vertical and horizontal mullions after the panels are installed and are therefore attached to the vertical and horizontal mullions by fasteners. Typically these fasteners are threaded members such as screws that secure the pressure plate to an external region, such as a mullion tongue of the curtain wall. This assembly step is generally time intensive and requires skilled manual labor. Moreover, the manual labor generally occurs outwardly of the curtain wall and therefore must be carefully designed to facilitate ease and reliability as well as some degree of accuracy and rapidity in installation. For this reason, numerous mullion tongue screw-race designs have been created over the years for facilitating the installation of threaded fasteners into the curtain-wall mullion. Two examples of prior art curtain wall mounting systems are shown in U.S. Pat. No. 5,592,795 and U.K. Patent Application GB 2 133449A.
Referring now to FIG. 1, there is shown an end elevational cross-sectional view of a curtain-wall mullion of the type having an external tongue adapted for receiving a pressure plate thereagainst and threaded fastener therein and comprising a part of a pressure-plate mounting system. A threaded fastener 122 in this particular application is a screw combining a drill bit tip and contiguous threaded body region that facilitates an assembly process. The threaded fastener 122 can be used with an automatic drill and positioned to both penetrate an otherwise solid surface and secure itself within a hole generated therein. The reliability of the connection with a threaded fastener such as the threaded fastener 122 in a curtain wall is then dependent upon a structural interconnection between an external tongue portion of the curtain-wall mullion and a threaded body portion of the threaded fastener.
Problems associated with the assembly process described above include the time necessary for penetration of a mullion tongue screw-race, as well as any tendency of the threaded fastener to “walk” when initially being driven into the curtain-wall mullion tongue. This may occur with certain screw-race designs. Since the threaded fastener 122 is typically manually aligned and driven by a hand-held drill supported by a skilled laborer outside a vertical curtain wall, the possibility of some screw movement (i.e., “walking”) is often tolerated. This means that the most desirable screw position may not be achieved. Secondly, the length of time necessary to penetrate a thick web portion of a screw race of an external mullion tongue further delays the process, decreases project efficiency, increases the possibility of associated errors and/or risks and is more time and labor intensive. The thickness and location of the web portion of a screw race may vary, depending on the design.
For example, referring now to FIG. 3 there is shown yet another prior art embodiment of curtain wall construction wherein another curtain-wall mullion tongue design is utilized for pressure-plate mounting. This particular tongue design incorporates a web portion that is at the lower region of a screw-race yoke and that is not designed to be penetrated at all. The yoke portion of this particular screw race comprises a pair of generally parallel lips that are spaced one from the other to receive a screw at any location therealong. The screw used therewith will penetrate the internal areas of the lips of the screw race to be secured therein but not extend beyond a given dimension that is less than the depth of the screw race. No web portion must be penetrated, which increases the speed of installation. However, the screw threads are only engaged on opposite side surfaces thereof and the pull out strength of the connection is dependent upon the lip engagement. Should the lips expand during loads, the pull out strength can be significantly reduced.